I have fathered 3 sons. Even in that case I would say they are my seed, not my seeds. Seed can be collective as well as singular.
The same is true of the Hebrew word. Yet Paul makes the grammatical distinction in specifically denoting it's singularity in Genesis 15 and 17 (the verses he quotes) to drive home the point that when God said the word "seed" to Abraham in Genesis 15:5, he was talking about Jesus.
[Gen 15:5-6 KJV] 5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. 6 And he believed in the LORD; and he counted it to him for righteousness.
[Gal 3:6, 16 NASB] 6 Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. ... 16 Now the promises were spoken to Abraham and to his seed. He does not say, "And to seeds," as [referring] to many, but [rather] to one, "And to your seed," that is, Christ.
So what we see is that it isn't that Abraham believed God when God said any random thing... Abraham believed God SPECIFICALLY when God was talking about Jesus Christ. So then from Galatians 3:16, we know that Genesis 15:5 was God preaching The Gospel of Jesus Christ to Abraham, and when Abraham believed the gospel, he was made righteous. Indeed, that is the very point and context to Galatians 3:16:
[Gal 3:6-9 NASB] 6 Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. 7 Therefore, be sure that it is those who are of faith who are sons of Abraham. 8 The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, [saying,] "ALL THE NATIONS WILL BE BLESSED IN YOU." 9 So then those who are of faith are blessed with Abraham, the believer.
But Paul goes even further in a mirror passage in Romans 4:
[Rom 4:9-13, 16-17 NASB] 9 Is this blessing then on the circumcised, or on the uncircumcised also? For we say, "FAITH WAS CREDITED TO ABRAHAM AS RIGHTEOUSNESS." 10 How then was it credited? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised; 11 and he received the sign of circumcision, a seal of the righteousness of the faith which he had while uncircumcised, so that he might be the father of all who believe without being circumcised, that righteousness might be credited to them, 12 and the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham which he had while uncircumcised. 13 For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith. ... 16 For this reason [it is] by faith, in order that [it may be] in accordance with grace, so that the promise will be guaranteed to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all, 17 (as it is written, "A FATHER OF MANY NATIONS HAVE I MADE YOU") in the presence of Him whom he believed, [even] God, who gives life to the dead and calls into being that which does not exist.
So just like the Gal 3:6-9 passage we see the purpose of faith itself being described. The purpose for faith is NOT to qualify one for righteousness, but to qualify one as a descendant of Abraham. God has ALREADY given a promise that the descendants of Abraham would all INHERIT the righteousness given to Abraham. However, here God is saying that it is not only those who are of the circumcision who qualify, but also those uncircumcised persons who have the same faith in the gospel of Jesus Christ as Abraham who qualify as Abraham's descendants and therefore heirs of the world.
These two branches of Abraham's offspring - one by the woman who is the covenant of The Law, and one by the woman who is the covenant of faith - are what Isaiah spoke of:
[Isa 54:1 NASB] 1 "Shout for joy, O barren one, you who have borne no [child;] Break forth into joyful shouting and cry aloud, you who have not travailed; For the sons of the desolate one [will be] more numerous Than the sons of the married woman," says the LORD.